High-yield catalyst components for the polymerization of olefins and in particular for ethylene are known in the art. They are generally obtained by supporting, on a magnesium dihalide, a titanium compound normally chosen from titanium halides, alkoxide and haloalcoholates. Said catalyst components are then used together with an aluminum alkyl compound in the polymerization of ethylene. This kind of catalyst components, together with the catalysts obtained therefrom, are largely used in the plants for the (co)polymerization of ethylene both operating in liquid phase (slurry or bulk) and in gas-phase. However, the use of the catalyst components as such is not completely satisfactory. Indeed, due to the high reactivity of the ethylene, the kinetic of the polymerization reaction is very high. As a consequence, the catalyst is subject to a very strong tension during the initial stage of polymerization which can cause the uncontrolled breakage of the catalyst itself. This event is the cause of the formation of fine particles of polymer, which, in turn, leads to low bulk density of the polymer and to operating process difficulties.
In the attempt to solve these problems, the catalyst is pre-polymerized under controlled conditions, so as to obtain pre-polymerized catalysts purposed to retain better morphology. In fact, it is believed that after the pre-polymerization, the catalysts increase their resistance in such a way that the tendency to break under polymerization conditions is decreased. As a consequence, the formation of fine particles would be reduced and the overall bulk density improved. Typically, the catalysts are prepolymerized with one olefin which is often the same olefin which is then polymerized in the main polymerization step. Accordingly, the non-stereospecific catalysts used for ethylene polymerization are often pre-polymerized with ethylene. The pre-polymerization with ethylene of a non-stereospecific catalyst to be used in ethylene polymerization however, does not fully solve the problem. In fact, the morphological properties and resistance of the catalyst are improved, if any, only in combination with an unacceptable decrease of the catalyst polymerization activity. This is confirmed for example by U.S. Pat. No. 4,325,837 which discloses in table 14A and 14B, the use of a non-stereospecific catalyst pre-polymerized with ethylene to an extent of lower than 50% b.w., based on the weight of the total pre-polymerized catalyst. The activity and the morphological properties of the polymer produced with the pre-polymerized catalyst are in certain cases lower than that of the non-pre-polymerized one. In the same patent it is said (column 37 lines 57-60) that the use of a pre-polymerized catalyst is of no advantage with respect to the non pre-polymerized one.
WO01/85803 describes that by the pre-polymerization of a non stereospecific catalyst with prochiral monomers it would be possible to obtain a catalyst for the polymerization of olefins capable to produce polymers with high bulk density and that has an activity which is higher than that of the original non pre-polymerized catalyst. However, the described pre-polymerized catalyst, although of interest under certain polymerization conditions, does not show an acceptable morphological stability under polymerization carried out under drastic conditions such as those carried out in the presence of high amount of hydrogen in order to prepare ethylene polymers with low molecular weight. Moreover, it has been noticed that the propylene pre-polymerized catalyst tends to form a too high amount of aggregates of particles that have to be removed in order to have a homogeneous lot with a consequent loss of material.
It has also been discussed in the prior art the option to use more than one monomer in the prepolymerization stage. The preferred option is that of using mixtures of different monomers polymerized simultaneously thereby producing random pre-polymers. EP-A-435332 is an example of such teaching describing the preparation of a prepolymer obtained by polymerizing small amounts of ethylene and propylene. The prepolymer/catalyst system is then used in the preparation of propylene heterophasic copolymers.
Sequential pre-polymerization of different monomers is also taught although limited to certain specific types of monomers. EP-A-604401 discloses the preparation of a pre-polymerized catalyst obtained by contacting, under polymerization conditions, a catalyst system with (1) a linear olefin and (2) a non-linear olefin in order to form a linear olefin/non linear olefin block copolymer as a prepolymerized solid product. This prepolymerized catalyst, after having been subject to a titanation treatment, is then used in the polymerization of propylene. Ethylene and propylene are listed among linear olefins, while saturated ring containing hydrocarbon monomers, branched olefins and aromatic monomers are included in the definition of non-linear olefins. Comparative examples carried out without pre-polymerization stage show that the specific prepolymerization treatment has no effect on catalyst activity or morphological stability while the major impact is on mechanical properties of the final polymer product.